This invention relates to the centrifugal casting of iron pipes. More specifically, it concerns the De Lavaud casting process in which (i) molten iron is poured into a revolving mold through a pour channel fed by a casting ladle, (ii) a relative longitudinal motion is produced between the channel and the mold, and (iii) the supply of molten iron in the channel is cut off before the casting nose reaches the end of the mold.
As is well known in the art, the pour channel is shaped like a gutter extended at the upstream end by a widened spillway gate into which the ladle pours the molten iron. The channel may be of a single piece or may be constructed of several sections reinforced by a support cradle. The mold generally has a socket end and a plain end, the latter being closest to the channel.
Italian patent No. 430,464 describes an apparatus for centrifuging metal pipes in which it is possible to manually adjust the slope and height of the casting channel prior to casting. These are set before undertaking a series of castings, and during each casting the slope and height of the casting channel are fixed and do not vary. Such apparatus enables adaptation to the various types of pipe manufactured, but does not allow for the correction of the flow speed when the supply of liquid metal is cut off. Nor does it ensure that the channel will be completely empty at the end of the casting.
In order to hold constant the flow of molten iron entering the mold, conventional processes alter the speed of relative axial translation between the mold and the channel, which involves a complicated control arrangement. Furthermore, near the end of the casting and after the supply to the channel has been interrupted, this speed of translation must be reduced, presenting the drawback of prolonging the length of the pipe manufacturing cycle.
Similarly, varying the flow of metal poured into the mold by changing the tilting speed of the ladle, or, more generally, the rate at which the channel is supplied, involves relatively tricky and complicated controls whose effect is delayed by the great length of the pour channel.
In addition, in the two preceding cases, a residual amount of molten iron remains in the channel at the end of the casting and hardens there in the form of an iron tab, which must be removed before the next casting.
In order to cure these drawbacks the assignee previously developed, as described in French patent No. 2,459,698, an apparatus which enables a simple and precise modulation of the flow of iron into the mold by varying the angle of inclination of the pour channel. According to the '698 patent after the supply of molten iron is interrupted the slope of the channel is gradually increased so as to keep the flow of iron constant until the casting nose exits the mold. This makes it possible to empty the channel nearly completely with each casting, which almost totally eliminates the drawbacks due to the residual iron mentioned above. This process thus makes it possible to obtain cast pipes of essentially uniform thickness without altering the relative speed of translation or the rate at which molten iron is supplied to the pour channel. To carry out this function, the '698 patent uses a nearly horizontal, cantilevered beam or cradle on which a channel for bringing liquid metal into the revolving mold is pivoted, with the point of articulation of the channel being located near the casting nose. A piston and cylinder unit placed below the beam on the opposite end from the casting nose (in the vicinity of the spillway) makes it possible to tilt the channel toward the revolving mold in the final phase of pipe casting in order to allow the residual quantity of molten iron in the channel to flow completely into the mold, while simultaneously maintaining optimal flow conditions.
Although the improvements realized by the '698 patent are satisfactory, several drawbacks still exist. For example, as the channel slopes toward the mold, which is simultaneously being driven in rotation and in longitudinal translation, the end of the casting nose approaches the lower generatrix or bottom of the mold because it is cantilevered from the channel's point of articulation. The cantilevering cannot be avoided because of the unacceptable overheating to which the channel journals would be subjected if they were installed at the very end of the casting nose. The result of this movement is a variable height of fall for the molten iron, which interferes with the distribution of the iron emptied into the revolving mold. The uniform spreading of the iron thus decreases as the casting nose approaches the inner wall of the mold, leading to the formation of pipes having a slightly wavy inner surface. The waves become more pronounced as the height of fall of the molten iron is reduced.
At first glance it may seem sufficient to increase the initial height of the casting nose to cure the above-mentioned drawback. However, it is also well known that in a pipe centrifuger the mold, or its protective inner coating, erodes more quickly the higher the height of fall of the molten iron. Such erosion constitutes a considerable drawback by reason of the higher manufacturing cost it entails.